The present invention relates to package-strapping apparatus, of which the strap in particular is a thermoplastic tape that is fed from a conveyor by means of a cross-sectionally U-shaped strap guide frame to a locking unit, a strap outlet aperture of said frame being at least partly closed by a first strap retainer device in order to preclude premature strap issuance from said frame, said retention device opening when the strap is pulled back to be tensioned around the package and to allow drawing the strap out of its outlet aperture.
Such strapping apparatus are pertinently known already in the state of the art and illustratively include the German patent documents 199 12 940 A1 and 25 07 717 C3 and the European patent document EP 0 738 658 B1.
The strap guide frame of such strapping apparatus typically comprises several legs associated with the said strap retainer device. The state of the art essentially discloses two designs of said retainer devices. In a first design the said retainer device is constituted by several cross-sectionally L-shaped flaps or lids bilaterally configured on at least one guide frame leg. Such lids are displaceably mounted at said guide legs and are approximately as long as the associated legs. In their rest position, they are biased toward said frame by retaining springs and they cover the strap outlet aperture. When the strap is pulled back in order to be tensioned around the package, said lids either are opened in automatic manner or by an external control, and thereby in their open position will impulsively make available the strap segment corresponding to their length.
In a further design of the state of the art, the strapping retainer device is constituted by lip-shaped flexible or plastic elements of defined cross-sectional shape which also are configured bilaterally in the region of the strap outlet aperture and which close said aperture. As described in the German patent document DE 199 12 940 A1, when being pulled back to be tensioned around the package, the strap displaces the zones of said cross-sectionally shaped elements only in its discharge zone, said zone during said process migrating around the strap guide frame.
The strap retainer device meets several purposes. On one hand it is meant to preclude premature and uncontrollable strap issuance from said outlet aperture when the strap is inserted into the said guide frame. As regards vertical strap guide frames, moreover, said retainer device supports the strap inside the guide frame against gravity.
Another significant function of the retainer device is to guide the strap when being pulled out of the strap outlet aperture during pull-back and tensioning around the package. Ideally the strap should be precisely situated within an accurately defined strapping plane on the package. In principle too the lids and the flexible, cross-sectionally shaped strips adequately assure such positioning, however various details are disadvantageous.
As already mentioned above, the said lids always uncover a relatively large portion of the strap outlet aperture, as a result of which strap issues from said zone in uncontrolled manner. When said lids are very long, the strap may come to rest on the package outside the strapping plane, and strapping of the package will be oblique and inadequate to keep said package together. The state of the art furthermore includes a procedure using several short lids, in fact minimizing the above problem, though the strap still may occasionally exit in uncontrollable manner the various zones of its outlet aperture and moreover it tends to twisting whereby it will not always come to rest flat on the package surface. When stacking packages, twisted straps can entail stacking instability. A plurality of small lids also incurs the drawback of requiring substantial maintenance. Each of such small lids is connected by an articulating fastener to said frame and comprises wear-susceptible retaining springs that must be changed frequently. Another drawback is the substantial noise generated by each lid hitting the strap guide frame when said strap outlet aperture is closed again.
The problem of uncontrolled strap pull-out from said strap's aperture does not arise when using the above cited textile lips or plastic, cross-sectionally well defined elements. In fact only the zone from which the strap is being removed will open the cross-sectionally well defined elements resting under tension against the strap applying a force directed in the direction of pulling on said strap and hence allowing removing the strap in controlled manner from the strap outlet apertures. Considering the present-day strap pull-back and pull-out rates occurring when the strap is tensioned around the package, said elastic, cross-sectionally well defined strips wear comparatively fast, again entailing higher costs of maintenance.